Microfilm to paper printing machines may include a light beam sequentially passing through an optical path including the microfilm, an objective lens, a complementary pair of zoom mirrors, adjustable in tandem, an image slit and the image plane surface of a photoconductive printer drum. The magnification of the optical system is dependent upon the optical path distance from the film plane to the image plane for a specific fixed focal length objective lens. This distance for the optical path from the film plane to the image plane, known as the total conjugate length (TCL), can be changed by selectively reciprocally moving the pair of zoom lens mirrors relative to the film plane and the image plane. However, for an objective lens having a fixed focal length, the amount of magnification capable from the system is a function of the amount of movement possible for the complementary zoom mirror pair. This zoom mirror assembly movement is limited by the amount of room available in the console or housing of the printing machine.
If additional magnification is required or desired for the system, the objective lens in the system is normally removed from the console and replaced by an objective lens with a different focal length selectively providing the magnification ratio range desired. However, for machines handling large paper sizes requiring large magnification ratios, the objective lenses needed as replacements are relatively large, cumbersome and heavy. The operator has difficulty in physically storing and changing the objective lens and in properly positioning the replacement lens in and along the optical axis of the system.
The restricted sizes of the consoles for printer machines and the size of replacement lenses have been factors limiting the production of practically sized and easily used printing machines capable of printing copies up to 36 inches wide, with magnification ratios running from 7.4.times. through 32.times.. The size of the latent images optically being transferred onto the printer drum also may create problems in copy quality and masking.
In this regard, the masking of the margins of the microfilm image being copied is conventionally done along the optical axis upstream of the image slit leading to the image drum by a high speed shutter and by separate side masks. The aperture opening controlling the amount of light exposure on the photoconductive drum is normally increased or decreased by a pair of pivotally mounted parallel rods being positioned upstream of the image slit, with pivotal movement thereof controlling the effective opening or aperture therebetween. With large magnification ratios for printing onto large width paper, the positioning of the shutter, masks and aperture control upstream of the image slit plane of the optical system may decrease the quality of the print being made and may affect the accurate positioning of the marginal edges of the print.